The recently completed Large Hadron Collider, the world’s most powerful particle accelerator and most ambitious scientific instrument, is being readied to address some of the deepest questions in physics (see “The Making of a New Collider”). Hundreds of feet below the surface of the earth, straddling the Swiss-French border near Geneva, it will smash counter-rotating, seven-trillion-electron-volt beams of protons against one another in a 27-kilometer ring of superconducting magnets.
With this immense energy, the LHC will be capable of producing new types of particles that are thousands of times heavier than the proton. And it will enable physicists to study phenomena at one-ten-billionth the scale of the atom. The science will be carried out with five multisystem particle detectors, the most massive of which are Atlas and CMS. Atlas is comparable in size to a seven-story building, 135 feet long and 75 feet wide; CMS, a somewhat smaller but heavier detector, weighs more than one and a half times as much as the Eiffel Tower. Each has about 100 million channels of electronic readout; with the accelerator, they constitute some of the world’s most sophisticated technology.
Why is the LHC so important? The standard model of particle physics, which has been experimentally confirmed with excellent precision at existing accelerators, was a major intellectual triumph of the 20th century. But the model is not complete, because it is based on a mass-generating mechanism that has not been verified experimentally. This is the so-called Higgs field, and one of the major objectives of the LHC is to confirm its existence or establish an alternative mass-generating mechanism. Theoretical calculations indicate that finding the Higgs particle, the quantum of the Higgs field, is well within reach of the LHC. Such a discovery would shed light on one of the great mysteries of nature: how mass is generated in the universe.
In this new energy range, there will be opportunities to explore physical principles and symmetries of nature that go beyond the standard model. In particular, we shall be able to search for signatures of supersymmetry, a phenomenon that has received much attention because it appears to be required in quantum theories of gravity and because it stabilizes the energy scale of the standard model against quantum fluctuations arising from processes in which particles flit in and out of existence. Supersymmetry assigns a mirror set of partners to the known fundamental particles, giving them the same electric charges but different spins. Theory suggests that the LHC is likely to be powerful enough to produce the lowest-mass supersymmetric partners. The lowest-mass uncharged supersymmetric particle is of particular interest, since it is an excellent candidate for the mysterious dark matter that makes up 23 percent of the total mass-energy of the universe.
Another question to be explored is the existence of extra spatial dimensions beyond the ones we know–a speculation prompted by string theory and by the observation that such dimensions could account for the weakness of gravity compared with the other fundamental forces. The LHC will also give rise to investigations of why the antimatter of the universe disappeared and matter remains. If this asymmetry were not embedded in our physical laws, we, and the universe as we know it, would not exist.
These and a host of other profound questions will be studied; but if history is a guide, the LHC will also turn up complete surprises, phenomena not anticipated by any theoretical speculation. The LHC will usher in a new era of discovery–findings that will stretch the imagination with the possibility of new forms of matter, new forces of nature, and new dimensions of space. It will give us a revolutionary new vision of the universe.
Jerome I. Friedman, Institute Professor at MIT, won the Nobel Prize for physics in 1990.
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